On the scatter in the relation between stellar mass and halo mass: random or halo formation time dependent?

The empirical HOD model of Wang et al. 2006 fits, by construction, both the stellar mass function and correlation function of galaxies in the local Universe. In contrast, the semi-analytical models of De Lucia & Blazoit 2007 (DLB07) and Guo et al. 2011 (Guo11), built on the same dark matter halo merger trees than the empirical model, still have difficulties in reproducing these observational data simultaneously. We compare the relations between the stellar mass of galaxies and their host halo mass in the three models, and find that they are different. When the relations are rescaled to have the same median values and the same scatter as in Wang et al., the rescaled DLB07 model can fit both the measured galaxy stellar mass function and the correlation function measured in different galaxy stellar mass bins. In contrast, the rescaled Guo11 model still over-predicts the clustering of low-mass galaxies. This indicates that the detail of how galaxies populate the scatter in the stellar mass -- halo mass relation does play an important role in determining the correlation functions of galaxies. While the stellar mass of galaxies in the Wang et al. model depends only on halo mass and is randomly distributed within the scatter, galaxy stellar mass depends also on the halo formation time in semi-analytical models. At fixed value of infall mass, galaxies that lie above the median stellar mass -- halo mass relation reside in haloes that formed earlier, while galaxies that lie below the median relation reside in haloes that formed later. This effect is much stronger in Guo11 than in DLB07, which explains the over-clustering of low mass galaxies in Guo11. Our results illustrate that the assumption of random scatter in the relation between stellar and halo mass as employed by current HOD and abundance matching models may be problematic in case a significant assembly bias exists in the real Universe.Comment: 10 pages, 6 figures, published in MNRA

The Number and Observability of Population III Supernovae at High Redshifts

We consider the feasibility of detecting Population III pair-instability supernovae (PISN) at very high redshifts with the James Webb Space Telescope (JWST). Four published estimates for the PISNe rate show a rather wide dispersion, between 50-2200 deg-2 yr-1. Correcting problems with several of these, we conclude that even a fairly optimistic estimate is probably a further order of magnitude lower than this range, at a rate of order 4 deg-2 yr-1 at z \~ 15 and 0.2 deg-2 yr-1 at z ~ 25, both with substantial uncertainty. Although such supernovae would be bright enough to be readily detectable with the JWST at any relevant redshift, the lower number densities derived here will likely require either a dedicated wide-angle search strategy or a serendipitous search. We expect that typically about 1 deg2 (or 500 JWST NIRCam images) per detected supernova at 4.5 mm must be imaged to detect one PISN at z ~ 15 and about 35 deg2 to detect one at z ~ 25. If some Population III star-formation persists to lower redshifts z ~ 5, then PISNe may also be detectable in wide-angle ground-based Z-band imaging surveys at ZAB ~ 23, at a density of order 1 deg-2 of surveyed area. In an Appendix, we consider the possible effects of intergalactic dust in obscuring high redshift supernovae or other high redshift sources. We show that the obscuration at a given rest-wavelength will peak at some maximum redshift and thereafter decline. While it may be a significant effect in observations of the very high redshift Universe, it is unlikely, even under rather pessimistic assumptions, to completely obscure primordial objects.Comment: To appear in the Astrophysical Journal (17 pages, 5 figures

Properties of Galaxy Groups in the SDSS: I.-- The Dependence of Colour, Star Formation, and Morphology on Halo Mass

Using a large galaxy group catalogue constructed from the SDSS, we investigate the correlation between various galaxy properties and halo mass. We split the population of galaxies in early types, late types, and intermediate types, based on their colour and specific star formation rate. At fixed luminosity, the early type fraction increases with increasing halo mass. Most importantly, this mass dependence is smooth and persists over the entire mass range probed, without any break or feature at any mass scale. We argue that the previous claim of a characteristic feature on galaxy group scales is an artefact of the environment estimators used. At fixed halo mass, the luminosity dependence of the type fractions is surprisingly weak: galaxy type depends more strongly on halo mass than on luminosity. We also find that the early type fraction decreases with increasing halo-centric radius. Contrary to previous studies, we find that this radial dependence is also present in low mass haloes. The properties of satellite galaxies are strongly correlated with those of their central galaxy. In particular, the early type fraction of satellites is significantly higher in a halo with an early type central galaxy than in a halo of the same mass but with a late type central galaxy. This phenomenon, which we call `galactic conformity', is present in haloes of all masses and for satellites of all luminosities. Finally, the fraction of intermediate type galaxies is always ~20 percent, independent of luminosity, independent of halo mass, independent of halo-centric radius, and independent of whether the galaxy is a central galaxy or a satellite galaxy. We discuss the implications of all these findings for galaxy formation and evolution.Comment: 28 pages, 15 figures. Submitted for publication in MNRA

Detection of galaxy assembly bias

Assembly bias describes the finding that the clustering of dark matter haloes depends on halo formation time at fixed halo mass. In this paper, we analyse the influence of assembly bias on galaxy clustering using both semi-analytical models (SAMs) and observational data. At fixed stellar mass, SAMs predict that the clustering of {\it central} galaxies depends on the specific star formation rate (sSFR), with more passive galaxies having a higher clustering amplitude. We find similar trends using SDSS group catalogues, and verify that these are not affected by possible biases due to the group finding algorithm. Low mass central galaxies reside in narrow bins of halo mass, so the observed trends of higher clustering amplitude for galaxies with lower sSFR is not driven by variations of the parent halo mass. We argue that the clustering dependence on sSFR represent a direct detection of assembly bias. In addition, contrary to what expected based on clustering of dark matter haloes, we find that low-mass central galaxies in SAMs with larger host halo mass have a {\it lower} clustering amplitude than their counter-parts residing in lower mass haloes. This results from the fact that, at fixed stellar mass, assembly bias has a stronger influence on clustering than the dependence on the parent halo mass.Comment: 6 pages, 4 figures, accepted for publication in MNRAS, Fig.4 update

Properties of galaxy groups in the Sloan Digital Sky Survey — I. The dependence of colour, star formation and morphology on halo mass

Using a large galaxy group catalogue constructed from the Sloan Digital Sky Survey Data Release 2, we investigate the correlation between various galaxy properties and halo mass. We split the population of galaxies in early-types, late-types and intermediate-types, based on their colour and specific star formation rate. At fixed luminosity, the late- (early-)type fraction of galaxies increases (decreases) with decreasing halo mass. Most importantly, this mass dependence is smooth and persists over the entire mass range probed, without any break or feature at any mass-scale. We argue that the previous claim of a characteristic feature on galaxy group scales is an artefact of the environment estimators used. At fixed halo mass, the luminosity dependence of the type fractions is surprisingly weak, especially over the range 0.25 ≲ L/L* ≲ 2.5: galaxy type depends more strongly on halo mass than on luminosity. In agreement with previous studies, the late- (early-)type fraction increases (decreases) with increasing halocentric radius. However, we find that this radial dependence is present in haloes of all masses probed (down to 1012 h−1 M⊙), while previous studies did not find any radial dependence in haloes with M≲ 1013.5 h−1 M⊙. We argue that this discrepancy owes to the fact that we have excluded central galaxies from our analysis. We also find that the properties of satellite galaxies are strongly correlated with those of their central galaxy. In particular, the early-type fraction of satellites is significantly higher in a halo with an early-type central galaxy than in a halo of the same mass but with a late-type central galaxy. This phenomenon, which we call ‘galactic conformity', is present in haloes of all masses and for satellites of all luminosities. Finally, the fraction of intermediate-type galaxies is always ∼20 per cent, independent of luminosity, independent of halo mass, independent of halocentric radius, and independent of whether the galaxy is a central galaxy or a satellite galaxy. We discuss the implications of all these findings for galaxy formation and evolutio

Dependence of the local reionization history on halo mass and environment: did Virgo reionize the Local Group?

The reionization of the Universe has profound effects on the way galaxies form and on their observed properties at later times. Of particular importance is the relative timing of the reionization history of a region and its halo assembly history, which can affect the nature of the first stars formed in that region, the properties and radial distribution of its stellar halo, globular cluster population and its satellite galaxies. We distinguish two basic cases for the reionization of a halo - internal reionization, whereby the stars forming in situ reionize their host galaxy, and external reionization, whereby the progenitor of a galaxy is reionized by external radiation before its own stars are able to form in sufficient numbers. We use a set of large-scale radiative transfer and structure formation simulations, based on cosmologies derived from both Wilkinson Microwave Anisotropy Probe (WMAP) one-year and WMAP three-year data, to evaluate the mean reionization redshifts and the probability of internal/external reionization for Local Group-like systems, galaxies in the field and central cD galaxies in clusters. We find that these probabilities are strongly dependent on the underlying cosmology and the efficiency of photon production, but also on the halo mass. There is a rapid transition between predominantly external and predominantly internal reionization at a mass scale of ∼1012 M⊙ (corresponding roughly to L* galaxies), with haloes less massive than this being reionized preferentially from distant sources. We provide a fit for the reionization redshift as a function of halo mass, which could be helpful to parametrize reionization in semi-analytical models of galaxy formation on cosmological scales. We find no statistical correlation between the reionization history of field galaxies and their environmen

The clustering of SDSS galaxy groups: mass and color dependence

We use a sample of galaxy groups selected from the SDSS DR 4 with an adaptive halo-based group finder to probe how the clustering strength of groups depends on their masses and colors. In particular, we determine the relative biases of groups of different masses, as well as that of groups with the same mass but with different colors. In agreement with previous studies, we find that more massive groups are more strongly clustered, and the inferred mass dependence of the halo bias is in good agreement with predictions for the $\Lambda$CDM cosmology. Regarding the color dependence, we find that groups with red centrals are more strongly clustered than groups of the same mass but with blue centrals. Similar results are obtained when the color of a group is defined to be the total color of its member galaxies. The color dependence is more prominent in less massive groups and becomes insignificant in groups with masses \gta 10^{14}\msunh. We construct a mock galaxy redshift survey constructed from the large Millenium simulation that is populated with galaxies according to the semi-analytical model of Croton et al. Applying our group finder to this mock survey, and analyzing the mock data in exactly the same way as the true data, we are able to accurately recover the intrinsic mass and color dependencies of the halo bias in the model. This suggests that our group finding algorithm and our method of assigning group masses do not induce spurious mass and/or color dependencies in the group-galaxy correlation function. The semi-analytical model reveals the same color dependence of the halo bias as we find in our group catalogue. In halos with M\sim 10^{12}\msunh, though, the strength of the color dependence is much stronger in the model than in the data.Comment: 16 pages, 14 figures, Accepted for publication in ApJ. In the new version, we add the bias of the shuffled galaxy sample. The errors are estimated according to the covariance matrix of the GGCCF, which is then diagonalize